The present invention relates to an electric power steering device for applying steering assist force to a steering mechanism of a vehicle by an electric motor.
There has been hitherto used an electric power steering device in which an electric motor is driven in accordance with a steering torque applied to a handle (steering wheel) by a driver to thereby apply steering assist force to a steering mechanism. The electric power steering device is equipped with a torque sensor for detecting a steering torque applied to a handle serving as an operating member for steering, and a target value of current to flow into the electric motor is set on the basis of the steering torque detected by the torque sensor. An instruction value to be applied to the driving unit of the electric motor is generated on the basis of the deviation between the target value and the detected value of current actually flowing into the electric motor. The driving unit of the electric motor comprises a PWM signal generating circuit for generating a pulse width modulation signal (PWM signal) having the duty ratio corresponding to the instruction value, and a motor driving circuit constructed by using a power transistor which is turned on/off in accordance with the duty ratio of the PWM signal, and applies the voltage corresponding to the duty ratio, that is, the voltage corresponding to the instruction value to the electric motor. Upon application of the voltage described above, the current flowing through the electric motor is detected by a current detecting circuit, and the difference between the detection value and the target value is used as the deviation for generating the instruction value. In the electric power steering device, feedback control is performed so that the current of the target value set on the basis of the steering torque flows into the electric motor.
Such a conventional electric power steering device as described above uses a torque sensor as an indispensable constituent element to set the current target value. However, the torque sensor is expensive, and thus it induces a problem that the cost of the whole conventional electric power steering device is increased.
Therefore, an object of the present invention is to provide an electric power steering device for setting a target value of current to flow into an electric motor without using any expensive torque sensor.
A first invention is an electric power steering device for applying steering assist force to a steering mechianism of a vehicle though a reducer by driving an electric motor in response to an operating amount applied by an operating member for vehicle steering, the electric power steering device comprising:
a steering angular-speed detector for detecting steering angular speed corresponding to rate of steering-angle variation caused by the operation of the operating member;
a rotational speed detector for detecting rotational speed corresponding to rate of rotational angle variation of the electric motor;
a target value setting unit for setting a current target value to be supplied to the electric motor;
a current detector for detecting current flowing in the electric motor and outputting the detected current value;
a controller for generating an instruction value for feed-back control to the driving of the electric motor on the basis of current deviation between the current target value and the current detection value; and
a driving unit for driving the electric motor in accordance with the instruction value,
wherein the target value setting unit includes,
a deviation calculator for calculating angular speed deviation based on the rotational speed, reduction ratio of the reducer and the steering angular speed, and
a target value calculator for calculating the target value of current by multiplying integration value corresponding to the operating amount, obtained by integrating the angular speed deviation, by a predetermined integration gain.
The angular speed deviation may be obtained by dividing value, which is obtained by multiplying the rotational speed by reduction ratio of the reducer, by the steering angular speed, or may be obtained by subtracting the rotational speed from value which is obtained by multiplying the steering angular speed by reduction ratio of the reducer.
According to the first invention described above, the value corresponding to the operating amount is calculated on the basis of a steering angular speed and a motor rotational speed by the target value setting unit. With this construction, even when there is not equipped any expensive torque sensor, the current target value corresponding to the steering torque can be set as in the case where an expensive torque sensor is equipped, so that increase in the cost of the whole power electric steering device can be suppressed.
A second invention is characterized in that the target value setting unit includes a compensation value calculator for calculating compensation value for phase compensation to the feedback control by multiplying the angular speed deviation by a predetermined proportional gain, and the target value calculator calculates the target current value by adding the compensation value to the value obtained by multiplying the integration value corresponding to the operating amount by the integration gain.
According to the second invention described above, the deviation between the steering angular speed and the motor rotational speed is multiplied by the proportional gain, and the multiplication result is added to the current target value, thereby performing the phase compensation to the feedback. With this construction, the phase compensation can be performed without using any expensive torque sensor. Therefore, increase in the cost of the whole electric power steering device can be suppressed, and also it is unnecessary to calculate a differential value for the steering torque for phase compensation, so that the phase compensation can be easily performed.
A third invention is characterized in that the electric power steering device according to claim 1 further includes a vehicle speed detector for detecting vehicle speed, wherein the target value calculator calculates the integration gain on the basis of the vehicle speed.
According to the third invention described above, steering assist can be suitably performed in consideration of parameters (for example, reaction of tire reaction, steering frequency range, etc.) varying in accordance with the vehicle speed.